The important bit here being that the custom element API simply doesn't allow declaring all relevant features of elements except their names. Whereas (SGML) DT

The important bit here being that the custom element API simply doesn't allow declaring all relevant features of elements except their names. Whereas (SGML) DTDs such as in any DTD for HTML before HTML5, elements were declared like this <!element img - - EMPTY> for elements with declared content EMPTY ("void elements" in HTML5 parlance), and <!element html O O (head,body)> as a basic example for tag omission/inference, where given these tag omission indicators (the letter O in the respective position), you can omit the html tags (and also head, and many other start- and end-element tags) when SGML can unambiguously infer those for you.